A LysR ‐family transcriptional regulator required for virulence in B rucella abortus is highly conserved among the α‐proteobacteria

Summary Small RNAs are principal elements of bacterial gene regulation and physiology. Two small RNAs in B rucella abortus , Abc R 1 and Abc R 2, are required for wild‐type virulence. Examination of the abc R loci revealed the presence of a gene encoding a LysR ‐type transcriptional regulator flanking abc R 2 on chromosome 1. Deletion of this lys R gene ( bab1_1517 ) resulted in the complete loss of abcR2 expression while no difference in abc R 1 expression was observed. The B . abortus bab1_1517 mutant strain was significantly attenuated in macrophages and mice, and bab1_1517 was subsequently named vtl R for v irulence‐associated t ranscriptional L ysR ‐family r egulator. Microarray analysis revealed three additional genes encoding small hypothetical proteins also under the control of VtlR . Electrophoretic mobility shift assays demonstrated that VtlR binds directly to the promoter regions of abc R 2 and the three hypothetical protein‐encoding genes, and DNase I footprint analysis identified the specific nucleotide sequence in these promoters that VtlR binds to and drives gene expression. Strikingly, orthologs of VtlR are encoded in a wide range of host‐associated α‐proteobacteria, and it is likely that the VtlR genetic system represents a common regulatory circuit critical for host–bacterium interactions.